Subnetting and variable length subnet masks (VLSM) allow for more efficient use of IP addresses through the use of different subnet masks. This reduces network traffic, optimizes performance, and makes management and geographic spanning easier. The document then provides details on IP address classes and their default subnet masks, an overview of binary subnetting, and an explanation of how VLSM can use varying block sizes and subnet masks to allocate addresses more efficiently than standard subnetting.